1. Field of the Invention
This invention relates to a power transistor and a semiconductor integrated circuit device using the same, and in particular, to such a transistor for use in the drivers of motors and actuators or in speaker drivers for audio systems that require electric current capacity.
2. Description of the Related Art
The structure of conventional power transistors is explained with references to FIGS. 7 and 8. The power transistor shown in FIGS. 7 and 8 is a vertically structured NPN-type power transistor formed within an epitaxial layer 72 of a P-type silicon semiconductor substrate. Vertical-type power transistors are the type in which the carriers that pass through the PN junction mainly diffuse in the vertical direction (the direction of the depth) of the transistor. FIG. 7 is a plan view showing the surface layout of a power transistor with one emitter region when viewed from above. FIG. 8 is a plan view showing the surface layout of a power transistor whose emitter region is divided into two by the base region when viewed from above. a low N-type impurity concentration. A base region 75 having P-type impurities is surrounded by the N.sup.- -type collector region 72a, and an emitter region 76 having N-type impurities is formed inside the base region 75. A base contact 74 is formed on the surface of the base region 75. The structure of the base contact is shown in FIG. 9, where the base contact 91 connects the metal wiring (not shown) with the base region 93 by forming a hole on an insulating layer 92 that covers the surface of the base region, allowing wiring material, such as aluminum, to flow into the hole.
In FIG. 8, the power transistor has an N.sup.+ -type collector region 83 having a high N-type impurity concentration arranged in a ring form surrounding an N.sup.+ -type collector region 82a with a low N-type impurity concentration. A base region 85 having P-type impurities is surrounded by the N.sup.+ -type collector region 82a. Emitter regions 86a and 86b with N-type impurities are formed inside the base region 85. A base contact 84 is formed on the surface of the base region located between the emitter regions 86a and 86b.
However, when a power transistor such as the one shown in FIG. 7 is used, a large current flows in the portion of the base region 75 near where the base contact 74 is formed, because the base contact 74 is in contact with the metal wiring layer. This is because the carriers in the circumference portion of the emitter region 76 near the base contact 74 are in an easy flow state ("easily activated state") compared with carriers in the center portion of the emitter region 76 far from the base contact 74. Also, when considering the flow of carriers in the vertical and lateral directions (the horizontal direction along the surface), the circumference portion of the emitter region 76 is closer to the N.sup.+ -type collector region 73 compared with the center portion of the emitter region 76. Because of this, the carriers in the circumference portion is more easily dissipated to the collector region 73 than the carriers in the center portion.
Therefore, the current density that flows from the emitter region 76 is non-uniform in some of the regions, causing heat to accumulate due to concentration of current, possibly causing safe operating area (SOA) destruction. In order to reduce current concentration in the emitter region 76 to prevent SOA destruction, the width b of the emitter region 76 is narrowed in some devices. But this will reduce the area of the emitter region 76 and result in a decrease in overall current capacity.
When a power transistor as shown in FIG. 8 is used, in which the overall emitter region area is enlarged by dividing the emitter region into two sections 86a and 86b, two regions A and B within the emitter region 86a and 86b (encircled by the broken lines in FIG. 8) are created, which areas are closest to both the base contact 84 and the collector region 83. As current flows from the metal wiring layer to the base region 85 via the base contact 84, carriers in the regions within the emitter regions 86a and 86b that are close to the base contact 84 are most easily activated. Because of this, large carrier flow will occur in the portions of the emitter regions close to this type of base contact.
In the aforementioned emitter regions A and B, these regions are in a state where the regions are activated to cause large carrier flow. When the emitter regions of the regions A and B are in such a state, the carrier flow is facilitated because the collector region is also in its proximity. Because of this, the current becomes concentrated in the emitter region in the A and B areas, making the area vulnerable to SOA destruction due to heat caused by the concentration of currents.